The present invention relates generally to leak detection systems for detecting fluid leaks from fluid storage tanks using distinctive tracer compounds to provide detectable components in a fluid leak from the tank. More particularly, the present invention relates to a system for automated soil gas sampling, analysis and reporting to determine the presence and magnitude of a fluid leak from a fluid storage tank. The present invention exhibits utility whether used to detect leaks in underground fluid storage tanks, aboveground fluid storage tanks or in fluid transfer pipelines. For purposes of clarity all such vessels shall be referred to as fluid storage tanks. The fluid stored in the fluid storage tank may be either a liquid, such as gasoline, or may be a gas, such as methane, natural gas, butane, propane or the like.
The present invention further provides a tracer leak detection method which relies upon the addition of a highly volatile liquid chemical to fluid contained within the fluid storage tanks. These tracer chemicals provide a unique and identifiable analytical signature. This signature is then used to detect and localize very small leaks from fluid storage tanks.
When a leak occurs in the fluid storage tank, the leaking fluid will contain a quantity of the tracer chemical. The tracer escapes from the fluid by vaporization and disperses into the surrounding soil by molecular diffusion. Soil gas samples are collected from the subsurface soil area by withdrawing a volume of soil gas through at least one sample probe disposed in an array in proximity to the fluid storage tank. Gas chromatography of the collected soil gas samples reveals the presence of the gas phase tracer, if any is present in the collected sample. The selection of tracer is important to insure that it provides a unique signature for gas chromatography.
The type of tracer chemicals useful in the present invention are more fully described in U.S. Pat. Nos. 4,725,551 and 4,709,557 issued to Glenn Thompson (hereinafter the "'551 Patent" and the "'557 Patent", respectively) the disclosures and teachings of which are expressly incorporated herein. Ideally, the selected tracer is normally a highly volatile organic tracer having a boiling point in the range of about -72.degree. C. to about 150.degree. C., with the preferred compounds being of the group known as fluorinated halocarbons, commonly referred to as halocarbons or fluorocarbons.
A wide variety of different soil gas sampling leak detection methodologies are known. Common to each of these methods is the provision of some means for collecting soil gas samples. For example in each of the '551 and '557 patents a sampling probe is vertically disposed in the backfill material surrounding an underground tank. The sampling probe has a plurality of apertures to permit soil gases to enter the probe for subsequent evacuation. It is also well known to employ carbon adsorbents in the sampling probe to collect hydrocarbons or tracer chemicals for subsequent collection by desorbtion from the carbon and analysis of the desorbed gas. Similarly, U.S. Pat. No. 4,754,136 discloses that a neutron back scatter gauge may be lowered into the sampling probe to determine whether the probe contains volatile organic material indicative of a leak from a fluid storage tank. A positive neutron back scatter reading is verified by running a gas chromatogram on a soil gas sample collected from the sampling probe and comparing the chromatographic signature with the known material in the fluid storage tank.
Each of these leak detection systems require that a soil gas sample be taken from the sampling probe then analyzed on a gas chromatograph. None of these systems, however, provide a means for continuously monitoring the status of the tanks. It has been found desirable, therefore, to provide a system for automatically and continuously monitoring the leakage status of a fluid storage tank.